I'm currently working on a project where I need to be able to switch 7m of 9W/m LED strip (connected from both sides) using an Arduino with PWM. So the power of the led strip is about 63W at 12V. I have a plenty of IRF540N mosfets lying around but I need to calculate if I need a heatsink, or not. The intended circuit looks like this:
Note that I don't want the high current to flow on the tiny PCB traces, that's the purpose of the screw terminal.
I know that a logic level MOSFET would be better, but if I understand the datasheet correctly, IRF540N should be able to switch even much higher currents with 5V at the gate than I need. Or would using e.g. IRL540N make any difference?
Do I need a heatsink? If so, how to choose one properly? Is there anything else I should be worried of considering my circuit?
Thanks in advance!
Best Answer
The IRF540 can have a gate threshold voltage of up to 4V according to the datasheet, so it would probably not be able to supply enough current for the LEDs to run close to 12V.Actually, taking a closer look at the datasheet for the IRF540, figure 3, it seems that it would be more than adequate of driving a 5A load at 5V gate drive. I would try this if i were you. The heatsink is still necessary though!
The IRL540 on the other hand is more suitable as it is a logic level MOSFET. According to the datasheet, the RDS(on) is around 77mΩ.
63W at 12V equals 5.25A, lets just say 5A as there will be a small voltage drop across the MOSFET, and that will lower the current in the LEDs.
The dissipated power in the MOSFET will then be:
\$ P_{dissipated} =RDS_{on} *I^2\ = 77mΩ * 5^2 = \textbf{1.9W}\$
According to the datasheet, the junction to ambient thermal resistance is 62 °C/W
This means the MOSFETs temperature will increase with
\$1.9W * 62 °C/W = \textbf{117°C} \$ from ambient temperature.
Assuming 25°C degrees ambient, that will result in 142°C. That is still in spec assuming ideal condition.. But you know, the world is not ideal..
EDIT: I forgot to take into account that the on-resistance has an significant increase with temperature, so you should definitely put a heatsink on it!